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Spotted aphids – clover or alfalfa?

In Australia, Therioaphis trifolli refers to two genetically distinct, host specific aphids, the spotted alfalfa aphid and spotted clover aphid.

 

Therioaphis trifolli is a distinct looking aphid species. Pale yellow-green in colour and about 2 mm long as adults, they have 4 to 6 rows of tiny black spots running lengthwise on their backs, which are just visible to the naked eye. The aphid is a pest of legume pastures, and a vector of some plant viruses including alfalfa mosaic virus (AMV) in medic and lucerne pastures, and bean yellow mosaic virus (BYMV) in clover.

But did you know that this species has two forms in Australia that are largely host specific? These are commonly known as the spotted alfafa aphid and the spotted clover aphid.   

Wingless spotted alfalfa aphid adults and nymphs (Source: cesar).

 

Spotted alfalfa aphid (SAA) was first detected in Australia in the late 1970s and heavily disrupted the lucerne-growing industry. At the time, the most commonly grown variety (cv. Hunter valley) was susceptible to attack by the aphid, and insecticides were heavily relied upon for control. The result of this was the evolution of varying levels of resistance to insecticides, primarily pirimicarb, but also dimethoate, omethoate and other older chemistries. Fortunately, these resistance issues largely appear to be a relic of the past with the introduction of SAA-resistant lucerne varieties and use of other integrated pest management strategies.

It was thought that Therioaphis trifolli was restricted to lucerne and some medics until large populations were found thriving on subclover in the 1990s. The aphid infesting subclover was not morphologically different from SAA, however subsequent research revealed it was genetically distinct biotype, and therefore was given the common name of spotted clover aphid (SCA).

Today, most lucerne varieties have some form of resistance against SAA. This has reduced the reliance on chemical controls for SAA, thereby reducing insecticide resistance selection pressures. Nevertheless, SAA is still exposed to insecticides in lucerne and medics when growers are seeking to target other pest species. This may be why the current pirimicarb label warns of insecticide tolerance in some SAA populations.

There is much less known about the SCA than SAA. There does not appear to be any insecticide resistance in SCA; its preference for clover pasture (as opposed to lucerne and medics) likely means that it hasn’t experienced the same insecticide selection pressure as SAA, and therefore historically hasn’t followed the same resistance trajectory.

 

Are you seeking further information? Visit our comprehensive spotted alfalfa aphid PestNote.

 

Thanks to Dr Peter Ridland (The University of Melbourne) and Dr Garry McDonald (cesar) for their assistance.

 

Field observations

Cody Stewart – IK Caldwell (Northern Country)  

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